ADAPTIVE ASYNCHRONOUS MEDIA SERVER REQUEST MECHANISM

Abstract: An asynchronous adaptive media server (16) mechanism permits a user to navigate around a modelled multimedia space as quickly as input events or instances are handled. The mechanism permits cancellation of outstanding queued requests that have become obsolete as the user navigates out of the area for which the request was needed. A client module (Fig. 8A) generates display requests by defining a client object for every active multimedia instance, and at least one server module (Fig. 8A) acts as a request queue manager and processes the requests. The server module (Fig. 8A) includes a media server (16) that preferably provides service of user-selected quality. Upon request cancellation, client ressources can be released immediately; a server state module mechanism may release server resources until there is no risk of endangering system operations. The ability to cancel or deserialize queued requests permits prioritizing the requests in a fashion that will permit media server (16) performance to be adaptively optimized to match the user's actions. The disclosed mechanism has application to any media that does not have a time-linear nature that maps directly onto some dimension of the modelled space displayed to the user.

Description: ADAPTIVE ASYNCHRONOUS MEDIA SERVER REQUEST MECHANISM FIELD OF THE INVENTION This invention relates to true multimedia server request mechanisms, and more specifically to a mechanism permitting prioritizing and cancellation of requests queued into such mechanisms to permit adaptive optimization of media server performance. BACKGROUND OF THE INVENTION As used herein, the term multimedia space refers to a dimension set that defines a domain navigable by a user. The multimedia space may have a single dimension (for example, a program timeline running from time start to time end) , or many dimensions (for example, a three dimensional map) . The multimedia space may be real- world, as in the case of an aircraft flight simulator, or it may be logical, as in the case of an n-dimensional virtual space representation. An essential aspect of multimedia systems is to provide the user with the ability to traverse a virtual space that is represented with text, still or short-sequence graphics, and some time-linear media (e.g. longer video or audio sequences) . The multimedia space to be navigated is typically data found in one or more media servers, for example a hard disk holding video, text, and/or audio information. Using a suitable interface such as a mouse, joystick or other mechanism, a user can navigate or traverse multimedia space by requesting displays on a video monitor. In a virtual reality application, moving a joystick to the right or left changes the displayed image from scenery on the user's right to scenery on the user's left. The joystick movement in essence requests display of the new scenery. Instead of using a joystick, the user could wear a motion-sensing apparatus such as a glove. Within a typical multimedia space, there are many dimensions for traversal or navigation, some of which map to time-linear media, and some of which do not. For a multimedia encyclopedia, for example, there is a linear dimension in the alphabetical ordering of items, and there is an X-Y space in the display of any given page. However, these dimensions have no mapping to the time- linear nature of a video or graphical sequence that might be displayed in association with a body of text about a given subject in the encyclopedia. Understandably, a user viewing a multimedia space wants the freedom to navigate along any direction and at any rate he or she chooses. Such freedom of navigation should not burden the user with real-world constraints of the media servers, for example, by having to wait excessively long for the pictures to be displayed. It must be recognized that the media servers containing the representations of the multimedia space have certain ballistic and linear constraints. Ballistic constraints concern how quickly data associated with a desired image can be moved from the media servers (hard disks, for example) to the system display engine. Video tape recorders, and many video servers, can provide quality video only within a small speed range around normal playback speed. Linearity constraints reflect the inability of a media server to truly accomplish random access. For example, one cannot randomly traverse different segments recorded on a video tape, and even a computer hard disk used for video playback and recording has limitations as to random access. Further, user request of certain recombinations of video material can result in further delays, so that the system can, for example, make video copies for use in special effects or to pre-render video sequences. Prior art multimedia representation mechanisms perform reasonably well, but only for multimedia spaces in which there exists a mapping of a time-linear media to a navigable dimension. However in the more generalized case where such mapping did not exist, prior art products and mechanisms perform poorly because they force the user interface to be locked synchronously to the media servers. In practice, these prior art mechanisms lock the sequence of user interface events synchronously to the display of the associated media. Because the display is considered to be a single timeline, it is navigable only at the rate of the slowest mechanism in the display path, typically the video server. A user requests displays (for example by moving a joystick) , but once a request enters the request queue, it must be displayed, and a given user request cannot be processed until all preceding requests in the queue are completed. Because prior art multimedia representation mechanisms employ a serialized, non-cancelable queue, system throughput is degraded if incorrect requests are queued. What would be desirable, for example, in a virtual reality simulator application, is for intelligent software to decide to pre-queue images representing adjacent pages of display, perhaps on the assumption that the user may next wish to see adjacent scenery. Unfortunately because prior art mechanisms do not permit cancelling uncompleted requests, the user would be forced to view such pre-queued scenery, even if the user had "walked" out of the area depicted in the pre-queued scenes. Thus, any efficiency that might be gained by pre-queuing is foregone because it would create an unalterable queue. Video editors constitute another area of application for multimedia display mechanisms. Video editors are used in video tape productions to combine selected video scenes into a desired sequence, such as special effects seen on television. To produce the sequence, the video editor must accurately frame-synchronize video tape recorders and peripheral devices. A user controls the editor from a keyboard while watching a monitor that displays information from the video editor. "Off-line" video editing systems are used to review source tapes, and to create simple editing effects such as "cut" and "dissolve". Such editors generate an intermediate work tape whose frames are marked according to an accompanying edit decision list ("EDL") that documents what future video changes are desired. By contrast, "on-line" editing systems are quite sophisticated, and control a variety of devices to produce complicated video effects. On-line editing systems are used to make post-production changes, including changes based upon the work tape and EDL from an off-line editor. The output from an on-line editing system is a final video master tape and an EDL documenting, at a minimum, the most recent generation of changes made to the master tape. A conventional EDL is a complex collection of timecode numbers and cryptic designations for keying, dissolve and other video effect operations. The timecode numbers give the precise time and video frame numbers where events occur on the finished tape, and also provide "in" and "out" times at which a given video source was transferred onto the finished tape. However at best, such conventional EDLs are a one-dimensional historical time record of the most currently issued commands that resulted in changes appearing on the finished video tape. In video (and audio) editing, the primary space being traversed is the program timeline that is being constructed, namely the EDL, which conventionally is shown graphically or textually as a timeline of edit- events on graphical editing systems. The media of primary interest is the time-linear program being constructed, whose description is the EDL. This is a special case within the general multimedia domain, because there is a direct mapping between the time-linear dimension of the media, and one dimension of the space being traversed, namely the EDL description for the program segment. Among video editing systems, the interaction between ballistically and linearly limited media servers distinguishes between linear and non-linear edit controllers. As noted, the linear and ballistic characteristics of video tape transports greatly reduce the speed with which a user can move about within the video representation of a program. As a result, application software rarely links the virtual program representation and the media. This constraint interferes substantially with the creative process. More flexible are non-linear systems, wherein the program timeline is typically locked to the timeline position of a textual or graphical representation of the program. Some commercially available editing products allow a user to navigate a timeline in a limited fashion to produce limited quality video images. Such mechanisms are relatively limited, especially with respect to the quality of the video provided. Because textual representations of EDLs are difficult to work with, manufacturers of off-line and on-line video editors have sought to embed video within the EDL. One such product is the AVID Media Composer, manufactured by Avid Technology, Inc. of Burlington, Massachusetts. This AVID product is an off-line video editor that embeds low quality video in the editor system. Figure 1 depicts the display 1 of an AVID system, wherein a first portion 2 of the screen is a current source window/monitor, a second portion 3 is the current program monitor, and third portion 4 presents a number of still video images that map directly to a displayed edit timeline 6 having discrete points 8. The images in regions 2 and 3 are sequences of images, whereas portion 4 presents various still video images. For a given length timeframe, a fixed number of still images 8 are displayed, interspersed uniformly along the timeframe. By means of a computer mouse, a user can click on a timeline point and display a corresponding video frame. In the conventional fashion, the AVID system binds the time linearity of the display media to the navigable dimension, namely the program timeline 6. Essentially such systems require that the display media be formatted in the same manner as the navigable dimension, thus creating arbitrary media events (timeline points) for the convenience of the system. In a broad sense, these two parallel timeline are then synchronously locked together. While the synchronous lock can prevent the navigation from proceeding faster than the display media, so doing can limit navigation. In practice, when the user moves along the virtual dimension at speeds in excess of the media server capability, the AVID system temporarily unlocks the two timelines and simply samples the media timeline, skipping video frames as needed. The sampling occurs as often as the server can handle, or only when the user stops at some point on the virtual timeline. In AVID-type systems, sampling provides a workable fallback mechanism, only because dimension mapping exists. The application software must issue but one request at a time, and issue the next request only after the first request has completed. When it is time to issue a request, the multimedia dimension reference point is mapped to the equivalent media timeline, and provides the sampling mechanism. Understandably, if no mapping existed, the application software could not pick a "next" picture without possibly leaving certain pictures blank, an unacceptable solution when a set of still pictures is to be displayed. No mapping exists because for still pictures, because such pictures are merely events not sequences. Sampling works for the window mechanisms depicted in Figure 1 as regions 2 and 3, although sampling can produce a somewhat jerky sequence because some stills in the sequence are omitted from display. For the stills in region 4, each individual picture must be displayed because there is no way to sample. That the limited AVID system works at all is because of the direct mapping between the time-linear media dimension and the navigable dimension of the multimedia space, i.e., the described program timeline. As noted, but for that mapping, the AVID system would not know how to sample the media timeline. This capability exists only because the AVID system has the direct mapping noted above. Because the AVID system maps one timeline position to one media position, the availability of the media position suffices. If the timeline position does not correspond to such media position, then by definition the requested timeline position is obsolete and may safely be discarded. Thus AVID-type systems can readily determine what requests are obsolete. Since the timeline can represent more data events than can be conveniently presented in a single display window, the AVID system displays one window that may be user scrolled left and right. However because the AVID model must first make a request for a still picture and then wait for that request to be completed, a user scrolling to the next page of timeline must first wait until all outstanding still video requests have been displayed. While this time delay may only be a few seconds, professional users scroll hundreds of times per day and often prefer to simply disable this feature to save time. Unfortunately for much multimedia there is no such mapping, and prior art systems such as AVID that rely upon the mapping breakdown completely. Assume for example that the user simply wants to display "head" or "tail" frames for events on the timeline, e.g., the first or last frames for a segment. If the user navigates the timeline faster than the media server can retrieve and display head or tail frames, no straightforward way exists to sample the media. The user wants to stop navigation at any point and expects to see all the pictures for the currently displayed region of program timeline. Because there is no mapping, sampling would omit pictures along the way, and leave blank pictures in portions of the display. Furthermore, since there is no linear dimension to sample, it would be difficult to develop a general algorithm that would never overburden the media server. Commercial products including the AVID system that are available for multimedia display, manipulation, and coordination address this problem forcing traversal of the multimedia space to be tied synchronously to the media servers. This forced synchronization makes such systems unbearably slow, especially for head frame display of a timeline, head and tail frame display of media segment libraries, and the like. As noted, in practice such systems are so slow that users prefer to turn off the media servers for these applications, sacrificing the additional information that a display might provide to obtain speed of navigation. While this may be an alternative for certain video editing systems, this alternative defeats the whole purpose of a true multimedia system by reducing the system to a mono-media representation of the virtual space. As an alternative, some prior art multimedia applications require the user to explicitly request the media within the display. Such applications can undesirably require more user interaction than is justified in many applications. Navigation can be slowed because the user is forced to make too many specific decisions. For example, the user may be forced to click on multiple icons before a preliminary decision can be made whether a displayed area is even an area of interest to the user. Further, because prior art multimedia display mechanisms synchronously couple the media server to the user interface, they lack the ability to pre-queue requests and then cancel queued requests, or to prioritize requests. For example, in a virtual reality display, as the user "walks" slowly past a building it may be desirable to render the building image with detail. However as the user begins to "walk" faster, it would be sufficient to render the building with less detail, and as the user "runs", to render the building with still less detail. Understandably rendering the building as a wire frame model useful for navigation requires less system resources (for example, video or graphics memory, computer resources or hard disk access, and thus time) than rendering the building in full detail. Because prior art systems cannot cancel a request, it is not feasible for such systems to issue both fast and slow requests for an object, and then (if desired) cancel whichever request has not completed before it is obsolete. By way of example, this constraint precludes requesting a wire frame image of a building and a fully rendered image for the same building, and then cancelling the fully rendered image if the user navigates past the building before the slower completing image displays. The request to display a wireframe image could be in response to a user joystick motion, or application software could detect that a "walking" velocity had carried the user past the building. Similarly, the cancel request could be in response to the user's walking past the building, thus rendering obsolete all requests to display images of the building. Clearly the inability to cancel such requests commits system resources longer than required in prior art systems. The result is considerably slowed navigation, or the requirement that all display requests be shifted to low quality. The flexibility to cancel outstanding requests and thus make better use of system resources and to expedite navigation is just not available in the prior art. In summary, there is a need for an asynchronous media server mechanism that allows the user to navigate a modelled space as quickly as input events are handled. The mechanism should recognize media events or instances as defined by the user, rather than imposing such definitions upon the user. The mechanism should permit cancellation of outstanding requests that have become obsolete as the user navigates out of an area for which the video was needed. Further, the mechanism should permit navigation that is independent of the ballistic and linear constraints of the media servers. Such mechanism should de-serialize the request queue such that media server performance is adaptively optimized to match the users' actions. The mechanism should be applicable to any media not having have a time-linear nature mapping directly onto some dimension of the modelled space displayed to the user. If applied to cases where such mapping does exist, the mechanism should provide a smooth sequence of high quality images, as contrasted with what is offered by prior art mechanisms. Finally, such mechanism should permit prioritizing requests according to system resources required to complete the request, and application-related priorities. The present invention discloses such a mechanism. SUMMARY OF THE PRESENT INVENTION For use with a computer system having a main computer and memory, the present invention provides an asynchronous adaptive media server mechanism coupled to the computer. A mechanism according to the present invention permits a user to navigate around a modelled multimedia space as quickly as requested input events or instances are processed, and can provide media display service of user- selected quality in response to the requests. The present invention includes a client module that generates access requests to the media server, and a server module that includes the media server According to the invention, each request defines a client object for every active multimedia instance. The media server queues the requests, which it processes as the requests reach the top of the queue. The media server also processes the requests and, unless cancelled, provides the requests to an output device. If the client module cancels a request that is being processed by the media server, client module resources associated with the request often can be released immediately. However, release of associated media server resources (and on occasion, client module resources) may be deferred until a critical state that could endanger system operation has been passed. More specifically, release of media server resources may be deferred until copying of associated information from the media server to the main memory has been completed. Communications between the modules is largely inter¬ module rather than inter-process. Because of its ability to be used in a true multi-tasking environment, the present invention includes a lock/unlock mechanism that provides inter-module mutual exclusion that makes atomic the module state changes during periods of mutual exclusion. In general, a lock is enabled whenever module state changes are made. Because it provides an asynchronous request mechanism, the client module includes a callback routine, for inter-module communication. The present invention permits queuing requests in order of a priority that can be associated with the amount of system resources required to complete the request and/or benefit to the user associated with completing the request. The present invention further permits re- prioritizing requests at any time there are in the queue, and cancelling requests at anytime in the queue. Thus, the mechanism permits cancellation of outstanding queued requests that have become obsolete as the user navigates out of the area for which the request was needed. The ability to cancel or de-serialize queued requests permits prioritizing the requests in a fashion that will permit media server performance to be adaptively optimized to match the users' actions. Because priority may be assigned in terms of server resources and/or in terms of user benefit, both high and low priority requests may be queued since obsolete requests may be cancelled before they are displayed. A mechanism according to the present invention has application to any media that does not have a time-linear nature that maps directly onto some dimension of the modelled space displayed to the user. Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 depicts an AVID off-line editor display, according to the prior art; FIGURE 2 depicts an on-line video editor with which the present invention is preferably practiced; FIGURE 3 depicts a graphical display of a program timeline, as displayed on an on-line video editor, according to the present invention; FIGURE 4 depicts the video form of a markfile display, as displayed on an on-line video editor, according to the present invention; FIGURE 5 depicts a text-only form of a markfile display, as displayed on an on-line video editor, according to the present invention; FIGURE 6 depicts the display for a session browser, as displayed on an on-line video editor, according to the present invention; FIGURE 7 depicts the display for an effects palette, as displayed on an on-line video editor, according to the present invention; FIGURE 8A is a state diagram depicting the normal full life cycle of a media request, and delineating client module domains and media server module(s) domains, according to the present invention; FIGURE 8B is a state diagram depicting the normal full life cycle of a media request, and demarcates client processes from media server processes showing in which modules the relevant code resides, according to the present invention; FIGURE 9 is a state diagram depicting the simple case where a picture moves off-screen before its request reaches the front of the queue, according to the present invention; FIGURE 10 is a state diagram depicting the case where the picture moves off-screen after retrieval has begun, but before client resources have been used, according to the present invention; FIGURE 11 is a state diagram depicting the case where the picture moves off-screen while retrieval is in a critical stage, after client resources have been involved in the transfer process, according to the present invention; FIGURE 12 is a flow diagram depicting the procedures following in responding to a received a media access request. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In overview, the present invention provides a media server-client model that handles asynchronous media service requests, and that permits the client to cancel any outstanding queued requests to the media server that have become obsolete. A single client preferably controls and coordinates distributed media servers. The user may navigate a virtual multi-media space as quickly as the application software can handle user-input events, the navigation being completely independent of the ballistic and linear constraints of the media servers. A very pleasing user interface results because the media servers can keep up (or very rapidly catchup) with the reference point in the virtual space. A model provided by the present invention has application to any media that does not have a time-linear nature that maps directly onto some dimension of the modelled space displayed to the user. Based upon client state changes, the client knows what region of the virtual space is displayed (or "active") at any time as the user navigates within in the virtual space. As the active region changes, client software recognizes media objects that have moved out of the active region. For those objects that are already displayed, the client generally releases some associated system resources (such as video memory) . However for those media objects for which requests to the media server(s) are still outstanding, the client cancels the requests. This effectively de-serializes the request queue, allowing still-valid requests to move-up in the queue. As a result, the media server can advantageously always be working on requests that are relatively current, without bogging down on obsolete requests. Further, the ability to cancel queued requests permits heuristic software to intelligently pre-queue requests, based upon anticipated user demand. The client adaptively optimizes the media server queue for any navigation path that the user follows. It will be appreciated that in a multimedia environment that includes networked or distributed media servers will especially benefit from the present invention, due to the transmission latency ballistics inherent in such environment. In one embodiment, requests are queued according to a priority that can reflect the request cost in terms of system resources and/or the user benefit of the request. Higher priority requests are displayed first that may obviate the need to continue processing lower priority, higher quality requests if it is cancelled. Further, by using a request ID that is associated with the request, the present invention permits re-prioritizing requests at any time they are within the queue. The present invention includes a client module that generates display requests by defining a client object for every active multimedia instance, and at least one server module that acts as a request queue manager and processes the requests. The server module includes at least one media server that preferably provides service of user-selected quality. Communications between the modules is inter-module rather than inter-process. It is not sufficient for the present invention simply to provide for making asynchronous requests to a separate process providing media services. If only asynchronicity were provided, the media server would quickly back-up and bog down with requests, as do prior art models. Even if the user slowed or stopped navigation within the virtual space, the media server(s) could take far too long to catch up with the user's reference point. Such delay would render the system nearly as unusable as prior art synchronous-based models. In the preferred embodiment, the present invention is implemented as an asynchronous video request mechanism that provides video context to a graphical or textual view of information. When used for video editing, for example, the present invention permits a more flexible user interface, and a more pleasing response than do prior art mechanisms, such as the AVID video system. Figure 2 depicts a modern on-line editing system, with which the present invention may advantageously be practiced. In the preferred embodiment, the on-line editing system is a model Axial 20-20 editor, manufactured by Accom, Inc. of Menlo Park, California. However, the present invention may be used with other systems, provided they are capable of true asynchronous (general multi-tasking or multi-processing) operations. With reference to Figure 2, the system includes a main computer and memory 10, coupled to user-input devices such as a keyboard 12, a trackball or mouse 14 (or joystick, pen, etc.), and to media servers 16, including perhaps hard disks 18 and optical disk(s) 20. The various media servers hold graphic, video, audio and text data to be displayed in a suitable format on a display monitor. The system can receive, in addition, video inputs that are provided to video imaging processing boards 24. Processed video signals are coupled from boards 24 to a video board and video memory unit 26 that in turn is coupled to a graphics board 28 that includes graphics memory associated with the hardware display engine. Graphics board 28 is also coupled to the main computer/memory 10, and provides video input to display monitor 22. As will be described more fully later, graphics board 28 provides drawable images to display 22, but can draw a color overlay atop the images. If the overlay is a color, the drawn image is not displayed, and if the overlay is the color "clear", the drawn image is visible to a user. This overlay of graphics atop video is in contrast to prior art graphical user interface implementations such as the Macintosh computer, wherein video often overlays graphics. In an on-line video editing application, the system may also include video tape recorders 30, video switchers 32, and effects processing equipment 34. In such applications, main computer/memory 10 can generate an edit decision list ("EDL") , or receive as an input an EDL prepared elsewhere, perhaps using an off-line video editing suite. The modules (or collections of code) comprising the present invention reside primarily within the main computer and memory 10, although other implementations could of course store the software elsewhere. The VFile module provides the media services to retrieve video stills from files stored on the media servers 16, hard disk 18, and/or removable optical disks 20. While tape drives or floppy diskettes could of course also be used, the associated latency would be unbearably long. The present invention uses X or XWindow system, a generic windowing software well known to those skilled in the art. For background, refer to Jones, Oliver "Introduction to the X WINDOWS System" (1989, Prentice Hall) , which is incorporated herein by reference. Applicants modified some device dependent portions of X to accommodate the described graphics system, a standard modification when porting X to specific hardware. The XClient software also is modified to work in a true multi-tasking environment. More specifically, the XClient event loop, required modification to accept input events from processes other than the XServer process. This requires that XClient must either perform a select on a variety of input pipes, or that a work process is added to the XClient event loop, which checks for messages from other client processes. All requests to the XServer must be channelled through the single XClient process. The reason for and nature of these modifications are known to those skilled in the relevant art and will not be described here. Is should be noted, however, that XServer itself is an asynchronous serial server that, upon receiving a picture request, must eventually draw the picture without possibility of cancellation. In the present invention, after XServer has been commanded to draw black at the proper screen location (e.g., to draw a black masking layer) , the video image is then formed but is not visible until X is later commanded to draw clear, thus revealing the image. XServer is a remote server that can be on a network. This enables the graphics applications to be drawn on a work station and send via Ethernet, whereupon a server will catch and draw the image. It should be noted that the term "client" is not necessarily synonymous with the application software. For example, XServer extensions have been proposed to provide bundled services of video windows. If such software were used, the application software would request a video window, and the server would take care of putting the video in a graphics window. In such a scenario, the XServer becomes the client of the video server mechanism. A brief overview of several aspects of the present invention will now be presented, a more detailed description appearing later herein. The Basic Mechanism to Display a Picture: A VWin module provides an interface to the hardware allowing the display of video on the display monitor 22, and an XServer provides graphic rendering services. To display an image on monitor 22, a client creates a VWin object, and binds this object to a graphic window within the X domain. In the preferred embodiment, the client draws the window as opaque, i.e., some color other than clear. The client requests the picture retrieval from the VFile server, and provides the VFile module with a callback routine and callback context (client state, usually a reference to the VWin/Xwidget object) . In return the client receives a request ID. When the VFile server has completed retrieval of the picture information into video memory 26, and the picture is ready for display on monitor 22, the callback routine is called with the client state. The client makes Xserver requests to draw the associated graphical window as "clear", whereupon the video is displayed on screen. As described elsewhere herein, one color within the X color space is interpreted by the graphics board 28 as "clear". When invoked, this feature allows video from the video display board 26 to show through a graphics overlay and be seen on monitor 22. The Asynchronous Nature of the Model: The present invention provides asynchronous media service requests in the following fashion. Requests to the VFile server make a change in the VFile module state, adding the request to a queue of outstanding requests. This occurs within the client process execution thread. (As used herein, "thread" shall be synonymous with "task" or "process".) The final step of the request mechanism makes the VFile process runable. All Vfile module states are protected with a mutual exclusion semaphore. While inter-process synchronization is implemented with a semaphore in the preferred embodiment, those skilled in the art will appreciate that a message or other inter- process synchronization mechanism could be used. The (lower priority) VFile thread calls back into the client module when the picture is available, this occurring within the VFile server process execution thread. The code in the client module sets up the requests to draw the graphics, and marks the client state to indicate that the picture is displayed. In the preferred embodiment, requests for drawing are actually requests to a window process that, in turn, issues X requests to the XServer. The embodiment serializes all X requests to the XServer, and provides a required mutual exclusion function not provided in standard X software (release 11, version 4) . Implementing De-serialization of Requests: In the present invention, de-serialization of requests is implemented as follows. When the reference point within the virtual space changes (usually but not necessarily in response to some user input action) , the client determines that a media object is no longer "active". Stated differently, the client recognizes that the point in the virtual space with which the media object is associated has moved out of the active or displayed region. In the preferred embodiment, the system recognizes that the user has scrolled the graphical timeline or segment library display, such that one or more stills effectively scroll off-screen. The client checks its state to determine whether the requested object was displayed, or whether the associated request to the file server is still outstanding. If the requested image was already displayed, then the video memory resources are preferably freed (making them available for other requests and other purposes) , and the object is deleted from the module state. In some applications, if the video memory resource is not needed elsewhere, the object may be retained in video memory. So doing would reduce the number of picture reloads as a user scrolls back and forth within one area. Understandably, implementing this further optimization to the asynchronous VFile mechanism involves the use of special memory management facilities. If the request is still outstanding, a call is made into the VFile module to cancel the request, the call occurring within the client process execution thread. This deletes the request from the queue of pending requests, with the result that the VFile process never sees the request. Implementing Multi-tasking Mutual Exclusion A request may be in process by the media server at the time a client wishes to cancel the request, and in fact the request may be in a critical state within the media server that must preclude the cancellation from proceeding normally. When this occurs, either the media server module or the client module must mark some state to indicate that the object is obsolete, and that the object should be released as soon as the critical phase is complete. If the critical phase is somewhere before the final copy into the client resource space (video memory 26 in the preferred embodiment) , then the server will handle the delete. From the client's perspective, the request is considered cancelled, and the client can free any client resources associated with the request. However, if the critical phase involves the use of any client resources, then the server will complete the request, and call back the client in the normal manner. In this instance, the cancel request fails, and the client marks its internal state to indicate that the object is obsolete. When the callback comes through, the client module callback routine sees that the object is obsolete. This callback runs in the media server execution thread. The callback handles the object deletion (freeing resources, etc.), rather than attempting to display the request. Understandably, it is important to carefully ensure the mutual exclusion of module resources. In a race condition, the object may become obsolete just as it becomes available for display. In such case, system deadlock can result unless rules are established and followed with respect to who may lock and hold what when performing critical inter-module operations. Request Prioritization and Re-prioritization The present invention permits prioritizing requests for queuing, and re-prioritizing requests at any time there are in the queue, using the request ID that is associated with a request. Re-prioritization involves moving requests to different locations within the queue, whereas cancellation involves removing the requests altogether from the queue. It is noted that a re-prioritization request on a request in process will generally have no effect. Generally, in actual implementation, a cancellation request may fail, whereas re-prioritization generally will not fail. A preferred embodiment of the present invention can recognize that different requests can involve different amounts of associated system overhead, and provides a mechanism for prioritizing the requests. For example, some requests may involve rapidly rendered, low quality wireframe images, whereas other requests require high quality, video memory intensive images that will take longer to render. The requests are preferably classified, with different requests providing different levels of quality, with different levels of associated overhead. The number of classification variants could vary from 2 to N, and is simply a matter of media server design. In this embodiment, the media server first services all outstanding lower quality requests (from any one client in a multi-client model) , before servicing any higher quality requests. Thus, if queued media events become obsolete, the more expensive requests will tend not to begin processing and thus provide a quicker response to new actions in the queue. This helps preclude the server from getting locked into time-expensive operations that will be canceled during request processing and enhances media server throughput. This feature is especially important when the user's navigation speed is just slightly faster than the media server(s) response capability. A series of requests for a media event, one request for each quality level available or desired, is issued. If necessary, the requests may be adjusted for the current navigation speed, to optimize the media server throughput. However, where the media server is implemented such that successive requests for higher quality of a given item can build upon previously completed lower quality requests, such client-side optimizing is unnecessary. One application for this embodiment involves picture data stored in a memory space, with all luminance bytes appearing at the front of a file, and the chrominance bytes stored later in the file (or in a separate file) . A black and white picture may be retrieved in half the time to retrieve a full color picture. However requests for both quality levels can advantageously complete in the same amount of time (with some negligible overhead) as a single high quality request. In this sense, the color bytes would simply augment the luminance bytes. Alternatively, the information could be stored not in pixel linear order but in modulo N order, where N is a sampling rate to reduce the quality (and associated data) for faster request throughput. A rendering server can provide a wireframe request in addition to a fully-rendered request. In this instance, the speed difference would be more marked, and the two requests would take somewhat longer to complete than completing only the higher quality request. In this application, a heuristic software associated with the present invention can decide to omit the low-quality request, perhaps according to the pattern of user input events. Figure 3 depicts the screen display 50 of a monitor 22 coupled to an on-line video editor provided with the present invention. A manufacturer of such editors in Accom, Corporation, located in Menlo Park, California. The display typically includes two video windows 52, with which the present invention is not directly concerned, a text display representing the current edit 54, a group of video context pictures 56, and a graphical timeline 58. Timeline 58 is a graphical view of the program material viewed along the x-axis of time. An element along timeline 58, element 58A for example, represents an edit, and any number of edits may exist on screen at one time. Edit 58A is highlighted in Figure 3 to indicate that it is the current edit, representing where the user is currently on the timeline. The user selection of the current edit may be made graphically, using the mouse or trackball 14 in Figure 2. The video context pictures 56 are representations of the edit elements of the graphical timeline. Thus, in Figure 3, picture 56A is highlighted to reflect that it is the current picture that corresponds to, or is the picture for, the current edit element 58A. The picture to the left of the current picture 5 A represents the edit to the left of the current edit 58A on the graphical timeline 58. In similar fashion, the other pictures of the video context are related to the elements of the graphical timeline. There are only seven video context pictures in the embodiment depicted, but there can be more than seven edit elements in the graphical timeline. Essentially, the video context window shows head frames around the current cursor context within the graphical timeline display. In the preferred embodiment, video context pictures 56 will be provided only for those seven edits centered around the current edit on the graphical timeline. For reasons of geometry the number of video context pictures displayed is seven for NTSC video format, and six for PAL video format. This embodiment provides a user with pictorial feedback on a graphical timeline, even if the editing machines are linear (tape transports, for example) . Users can edit visually, instead of having to rely on EDL numbers or text alone. Because the present invention does not bind retrieval of the video context pictures synchronously to the user navigation along the timeline, the user may navigate rapidly. Figure 4 depicts the video form of a markfile display 60, as seen on a monitor 22 coupled to an on-line video editor. Each element in the markfile describes a segment or piece of source material that can be used to build program material. For the video form of the markfile depicted in Figure 4, each element in the window consists of a picture and corresponding text. Figure 4 shows two rows of pictures 62, 66, beneath which are displayed two rows of text 64, 68. The pictures 62, 68 are still video images that relate visually to the source material. The windows in which the pictures fit can also be made to display live video images. The text 64, 68 describes each markfile element. As shown, the text preferably shows the time code, source reel number, an optional description (for example, "beach scene 3") , and key words (matte, background, fill, for example) . The present invention can parse or filter the comment string, sorting and selecting according to whatever terms the user has provided as key words. It is not necessary that the present invention have any understanding of what is meant by the user-provided comments. Understandably the pictures 62, 66 that will be displayed can be in any order, which precludes storing them in an optimum sequence for retrieval. It will be appreciated that the displayed media events have no mapping or linearity running along any navigable dimension, but are merely events that happen to occur at a point on the screen. Figure 5 depicts a text-only form of a markfile display, as displayed on monitor 22 coupled to an on-line video editor. The content of the text displayed is similar to what was depicted in Figure 4, except that each element is represented by one or more rows 70 of text. In Figure 5, row 70A is highlighted to indicate that it has been selected by the user. Figure 5 is similar to what might be displayed on a prior art system where, for ease of rapid navigation, corresponding video pictures were not displayed. Figure 6 depicts the display 60 for a session browser, as displayed on a monitor 22 coupled to an on-line video editor. A session browser is used to review short animations of program material for an editing session. The "session" is a large file representing the contents of a hierarchial database representation of an EDL, found in Accom on-line video editors with which this embodiment is preferably used. This database includes information as to edit history, devices used, the hierarchical relationship giving rise to the final edit (for example, a key overlying a picture to which the key is bound) . In this embodiment, program overview video sequences are used to represent an entire program, to permit rapid browsing among all of the stored edit-sessions. Each element in the session browser describes the program material for a single session. An element consists of a picture and a list of programs. In Figure 6, a row rows of pictures 80 is shown, beneath which is a row 82 of program lists. Using the trackball, mouse or equivalent device 14, the user can select a program from the list of programs. Animation will appear in the corresponding picture for the element selected. The present invention permits the user to rapidly browse (or navigate) through what may be a very considerable quantity of session material. Figure 7 depicts a display 60 of an effects palette, a display somewhat similar to that of Figure 4. Each element in the effects palette describes an effect. Each element consists of a picture that contains an animation for a given effect, and text that contains information about the effect. In Figure 7, two rows of pictures 90, 92 are depicted, beneath which are rows of text 94, 96. The pictures 90, 92 are preferably sequences of still images that animate the actual effect, but they may be stills as well. The pictures may, for example, be icons representing the effect, which icons when selected by the user demonstrate the effect. The effects palette binds the still picture or an icon in the sequence to the effect, a video roll for example. Because requested images may be cancelled without being displayed, the user is free to cursor off the display, producing an auto scroll that will display new media events. Other embodiments that can be achieved with the present invention include a heads display that augments a textual program display. While having similar utility to the above-described video context window application, a different format is used. Another embodiment provides a heads display for a layering timeline display, somewhat like the video context window, but using a layering- oriented screen layout. In yet another embodiment, head/tail displays are made for current source marks, the video server essentially providing a small swap-palette for "undo" operations, and for saved pages of marks. Having described several applications of the present invention, the operation of the invention will now be described. The present invention does not implement a procedural design but rather an Object Oriented design, in which various procedures flow through different threads. The operation of the present invention will first be described with reference to simplified state transitions, namely Figures 8A-11, which show simplified state transitions, and then with reference to the flow chart of Figure 12. Figure 8A is a state diagram depicting the normal full life cycle of a media request, according to the present invention. The various possible states, or events along the picture processing stage, are denoted A-F. As depicted in Figure 8A, at all times states A, B, C and D are within the domain of the media server module(s) , while states E and F and any user or other input event [1] are at all times within domain of the client module(s) . The media server module is also referred to herein as the VFILE media server module. Code for the present invention resides primarily within the VFILE media server modules (which encompass or encapsulate states A, B, C and D) . Code for the application software is contained primarily within the client modules, which encompasses everything not within the media server module(s) , according to Figure 8A. As used herein, modules refer to collections of code, and include a variety of data structures and define actions that can be taken with respect to the structures. According to the present invention, the modules are encapsulated in well defined user space that permits communication to be inter-module (as contrasted to inter¬ process) . By inter-module, it is understood that one or more than one processes can run through two blocks of code controlling two different sets of resources, whereas inter-process denotes that two processes can run in parallel through a single block of code. As used herein, the terms "thread", "task" and "process" are interchangeable. Thus, within the client module, event [1] occurs, moving the desired media item on the display screen of monitor 22 (see Figure 2) . Typically event [1] is defined by a movement of the mouse/joystick/trackball 14 (see Figure 2) causing a media item to move on the display screen into or out of an active region of interest. Those skilled in the art will appreciate that event [1] may more properly be defined as an event associated with an instantiation of a multimedia instance. Similarly, Figure 8A depicts the occurrence of a user input event (or instance) [2], which event or instance moves the media item off-screen. This off-screen movement renders the image request obsolete, and constitutes a user request to delete or cancel the request for the image. It is to be understood that event [1] (or "instance [1]") may in fact be provided by the application software rather than by a direct user movement. For example, in a virtual reality application, velocity can carry the user's reference point through space, without further user intervention. Motion of this reference point in turn can generate multimedia events. However for the sake of simplicity and with this understanding, event or instance [1] will be referred as a user input event or instance. In any case, with reference to Figure 8A, in event or instance [1] client software within the client thread detects that certain active events have moved across a boundary defining the active region. In state A, a picture is thus requested, the request entering the request queue but remaining inactive therein. As described elsewhere herein, it may be desirable to associate a priority with the request, permitting the request to be prioritized in terms of cost of server resources, user benefits, or a combination of each. Such prioritization would allow, for example, rapidly drawn wireframe images during rapid user navigation, which images are rendered more fully when the navigation slows, or black and white images during rapid navigation to transition to color images when navigation slows. From a client perspective, the flight simulator, for example, would pace higher priority on images along the line of trajectory, than on those along the side. From a client and user benefit, it would therefore be advantageous to place higher priority upon forward-looking images rather than side-looking images. Once state A occurs, event [2] may occur anywhere between states A and F. By way of overview, as will be seen with respect to Figures 8A-11, if event [2] occurs at states A or B, we proceed directly to state F. If event [2] occurs at state C, the state transition splits. The client thread and client module proceed directly to state F; the server thread and server module may wait to first complete state D before proceeding to state F. If event [2] occurs after state E (as shown in Figure 8A) , the full request life cycle has occurred, and we proceed directly to state F. However should event [2] occur between state D and state E, we cannot proceed directly to state F without potentially creating a conflict state. The state C to state D transition is depicted in Figure 8A with a somewhat lengthened path to depict that this transition can take a relatively significant amount of time, as typically a large block of data is copied into client resources. While this copying process is on¬ going, the client resources (e.g., video memory 26 (Figure 2) must be protected. Therefore a cancel request [2] that arrives before state D has completed is denied until completion of state D, whereupon the transition occurs directly to state F, as will be seen. With reference to Figure 8A, at state B, the requested picture is in the first stage of processing, and the appropriate portion of the media server 16, or hard disk 18 or optical disk 20 (see Figure 2) holding the image is read into the server memory. At state C, the requested picture is in the second stage of processing, and is copied from the server memory to the client video memory (see 20, Figure 2) . However according to the present invention, the image is drawn in black but is hidden beneath a colored graphics overlay until fully present. As noted, the state C to state D transition may relatively considerable time, and can represent a critical time wherein event [2] may arrive. In terms of time, the B to C transition can actually be longer, but does not represent a critical time period. It will also be that some implementations may omit state C, and proceed directly from a state combining states B and C, to state D. In Figure 8A, at state D the picture retrieval is complete and a callback is generated. At state E the display 22 is enabled (e.g., the colored graphics layer atop the image is changed to "clear") and the underlying previously drawn image (drawn at state C) is now visible on the display. At this point in Figure 8A, user input event [2] moves the media item off-screen, and state F depicts that the requested picture is now obsolete. At this state, all resources (for example video memory 28) are released. It will be appreciated that event [2] can occur other than after state E has been attained. As such. Figure 8A depicts the normal full life cycle of a media request. Figure 8B is similar to Figure 8A, but depicts the demarcation between the client process and the media server process. As noted, instance [2] may in fact occur before the requested picture has been displayed on screen (state E) . Figure 9, for example, depicts the best case for media server throughput in a heavy request scenario, namely where the user causes the picture to move off-screen before it has ever reached the front of the request queue. In Figure 9, the user input event [1] occurs as before. However before state B is attained, the user input instance [2] moves the media item off-screen. As noted, the off-screen movement renders the request for the picture obsolete, and constitutes a cancellation request. Because as yet no server resources have been committed to this request, the transition from state A may go directly to state F, and the cancel request succeeds. In Figure 9, states B, C, D and E are shown in phantom to indicate that while potentially these states could exist within their respective modules, they in fact are never entered according to the present invention. Figure 10 depicts the sequence of states for the condition where the user input instance [2] moves the media item off-screen after retrieval has begun (state B) , but before client resources have been used (state C) . In this instance, the transition is from state B to state F, the transition occurring in the client immediately, and occurring in the server as soon as the read from hard disk 18 (or media server 16, or optical disk 20, see Figure 2) completes or is interrupted. This is an important aspect of the asynchronicity of the present invention. The cancel request succeeds, and the client module is free to immediately release client resources. Again states C, D and E are shown in phantom, as these states are never reached under the scenario depicted in Figure 10. Figure 11 depicts the more interesting and critical case where the picture moves off-screen after state B but before state C has completed, which is to say after client resources become involved in the transfer process. In this critical case, the cancel request fails, and client resources are eventually freed by a callback routine. As will be described in detail later herein, unless this critical case is properly accounted for, the present invention can crash computer 10 or trash the images displayed on monitor 22. Figure 12 depicts the procedure of the present invention from the time a media access request is received 100 in response to a user event [1], through the display of the requested information 122 and the release of system resources 106. At step 100, a user event [1] creates a media access request that is received and placed in the request queue at step 102. Step 102 corresponds to state A, which was described earlier. If, at step 104, a cancel is received (from user event [2]), the procedure can go directly to state F, step 106, whereupon the cancelled request is removed from the queue, and system resources are released. If a cancel request has not yet been received, processing of the request continues and state B is entered. As shown at step 110, if a cancel request is received during state B, there is a transition directly to state F, whereupon the cancelled request is removed from the queue, and system resources are released. If a cancel request has not yet been received, processing of the request continues and state C is entered. As shown by step 114, if a cancel request is received during state C, a "kill request" is generated and at step 118, there is a transition to state D, whereupon retrieval of the requested information is completed. If during state D either the "kill request" or a cancellation from user event [2] was received, the process transitions to state F, whereupon the cancelled request is removed from the queue, and system resources are released. If neither a "kill request" nor a cancel request has thus far been received, then at step 122 there is a transition to state E. At this point the request is displayed, for example on output monitor 22 in Figure 2, the cancelled request is removed from the queue, and system resources are released. APPENDIX I, attached hereto, sets forth the source code implementing the various modules used in the present invention. Having described the scenarios depicted in Figures 8A-11, and the procedure flow depicted in Figure 12, the user events [l] and [2] will now be described from the perspective of the client module and from the perspective of the server module. The following detailed description will provide those skilled in the art with a full understanding of the operation of the present invention. Instance \ 1 ~ as viewed from the client module: What follows describes the normal procedure of two different threads proceeding through a single client module, and requesting services from other modules as needed. Steps (2)-(7) will represent the client module side of state A , which has been triggered by event [l]. Steps (8)-(13) represent what occurs when the client module server thread transitions to state E. (1) The user (or background update) takes some action to move the context within the multimedia space As noted, event [1] will be a user action (a mouse movement and click, a joystick movement, etc.), or a background update action, such as a perceived velocity change in a virtual space simulation application. Event [1] essentially moves the context within the multimedia space. (2) Client checks newly active screen region for instances [l] and receives a picture ID for each client object The client (which need not be synonymous with the application software) knows how much (more specifically, which region) of the virtual space is displayed (or "active") at any time, based upon the client state. The active region changes as the user navigates within in the virtual space, software detecting activation of the mouse cursor (or the equivalent) . The software further detects media objects that have moved out of the active region of the display. The client then checks the newly activated screen region for events. In representing the multimedia space, when the mouse causes a new region to be entered, that region is described in terms of text, and a pointer. As used herein, it is understood that the multimedia space may include more than pictures, sound and text, for example. The pointer is a picture identification ("ID") that, in the preferred embodiment, is implemented as a file name and a slot number within the client module. A header on the file says where the slot is physically located, the slot then mapping to an address in the hard disk where the associated file is located. Typically the picture ID is hard coded in a map containing IDs boundup with a library server for media. Alternatively, the picture ID may be described in a text file with locations and names, the text file being read at system bootup. Upon a client request, the library server is called for a picture and returns a picture ID. However the picture ID may be implemented, it suffices that the VFILE media server knows what it is retrieving. Alternatively, the user could specify a time frame or command a jump to a given video page where a mark file picture is expected to be found for a given video segment. In a preferred embodiment, the server module maps the video segment to a picture ID. Upon being given that picture ID, the VFILE media server knows where on the hard disk to find the picture. (3) For each media incident [1], client resources are obtained For each media incident [1], client resources must be obtained, which requires an allocation within the video output memory 26 (Figure 2) . (4) The state is locked, and for each media incident [1], a new multimedia client object is created To ensure inter-module mutual exclusivity, a lock is placed on the state, and for each media incident [1], a new multimedia client object is created and added to a client object list, and a reference is set to the video output memory. The lock could have been enacted earlier, but at a minimum must be enabled at this point. The lock state is carried with the object within the client object list. The enabled lock ensures that nothing will alter state tables until the multimedia object is completely created and an atomic update of the state relative to any other processes is complete. Associated with the client object will be a picture ID, a pointer to video memory output (pVOMem) , a pointer to VWin (the binding software object for XWidget that ensures that the proper video memory contents are put on screen to match Xwidget) , a request ID (a unique identifier returned by the picture transfer service) and two flags that are initially set to "no" . The first flag is labelled "Is Request Complete" and the second flag is labelled "Must Request Be Killed?". The client object also includes a pointer to XWidget that instructs how to draw the graphics window into which the picture will be drawn. Of course between steps 1-5, it is permissible to create other objects to draw into. In such instances, XWidget is created at the proper point on screen. If, for example, a requested picture were in the top display row and the user then did a half page scroll, the video request would not be cancelled, according to the preferred embodiment. Instead, XWidget would move, to which XWidget the client object simply has a reference, without requiring knowledge as to exactly where XWidget was located. Preferably, whatever moves XWidget (a scroll for example) is made to also move VWIN. Alternatively, VWIN can be drawn, and a query made into X to learn the location of XWidget for drawing at that location. So doing permits the state to move from XWidget to VWin, rather that having the client modify the VWin state. Thus, XWidget and VWin are bundled by the extensions to X such that VWin moves when XWidget is moved by the client, and is unnecessary to modify the XServer code. This is in accord with an underlying philosophy of the present invention that distinct, distributed servers that the client manages shall be employed. A less desirable alternative would be to use monolithic servers that do everything but demand substantial knowledge about the video window. (5) For each media incident [1], the picture transfer request service is called In this step, the client sets up a picture retrieval (or queue item) request to the media server module, after which the system proceeds upon receipt of a returned request ID from step (6) , but without having to await the actual picture retrieval. The request is made to the picture transfer request service whose file and slot number were designated in step (2) . The client provides the media server module with a callback routine and a callback context (client state, usually a reference to the VWin/Xwidget object) , and in return receives a request ID that identifies the requested picture and states how to get the picture out into the system resources. The request ID is an abstract identifier that uniquely identifies the particular multimedia instance [1]. The ID is intentionally abstract because the client cannot and should not do anything to the ID except pass it into the VFILE media server module. This abstraction promotes module encapsulation. Those skilled in the art will appreciate that because the present invention is asynchronous, a callback routine is required. In the preferred embodiment, the VFILE module maintains the callback pointer as part of the request object, and this function is called back at the appropriate time. In turn, this function will make a request to the XServer for the display of the requested image, using the client state passed in when the original request was made. The XServer provides a memory blast when writing within state E, when the display is enabled (e.g., overlay color changed to clear), and when XWidget is relocated. When the media server module completes retrieving the picture information into video memory, and the picture is ready for display, the callback routine is called with the client state. A pointer to the function that is the callback routine, and the client state, were saved associated with the picture request that was originally made. The callback routine is usually called for a completed action, and resides within a client module. The callback routine requires the picture request ID and/or the client state to determine which picture request has just been completed. From the client's perspective, what is seen in the callback is the client object created in step (4) . In short, at system initialization, the client module(s) create a client ID, and when requesting a picture, the client receives a picture ID and passes in the client ID so the VFILE media server module can identify the requesting client, and know how to handle all events for that client. A single callback procedure is used in the system, as contrasted with the use of a per-picture callback. As step (5) only involves media server module state resources or input/output, blocking on the picture retrieval is not required. A message is then sent to a separate server process advising that at least one additional item is now in the queue, which message causes the server process, if idle, to wakeup and act. The message is preferably implemented using a semaphore. Note that this inter-process communication occurs entirely within the server module. The client module need not know the details. In Figure 8A, the picture transfer request to display is the interaction between instance [1] and state A. More specifically, the picture transfer request service is called and is passed a client ID created by the server module, and a picture ID and client state. The picture ID advises how to find the picture, and the client ID advises how to callback the client, and the client state is passed back to the client callback to advise which picture has been completed. The client ID is used in preference to passing a new pointer to the picture transfer request service. Because the client ID includes a reference to a name, it is especially useful for error logging documentation purposes. Whenever the picture is done, the server module maps the picture to a client ID, which in turn is mapped to a callback to the client. This callback is called with the client context for the just completed request. Alternatively, the function could be passed in whenever a transfer request is made. In the preferred embodiment, the picture transfer is actually to resources that will make the request displayable. The client makes Xserver requests to draw the associated graphical window as "clear", whereupon the previously drawn video is displayed on screen. The client provides the VFile module with a callback routine and callback context (client state, usually a reference to the vwin/Xwidget object) . A preferred embodiment includes a segment library that can quickly put text description on screen for use in requesting a picture. The segment library maps many VFiles into an abstract collection for browsing, sorting, etc. When the request is made, the segment library provides the picture ID, whereupon the picture transfer request service fetches the picture. A layer of abstraction is provided that permits the client to indicate a desired timecode range, whereupon the segment library, which manages many files, can identify which file contains the corresponding images. A module provides the initialization routine where the list of files holding media is examined, and each file is opened to learn from the file headers what pictures are contained therein. The list goes to the library to update segments. When client has an event described, for example in a mark file segment sense, a list is constructed from which the associated pictures may be identified. (6) For each media event/incident [1], the request ID field of the client object is set When the request ID is returned, it is put into the client multimedia object created at step (4) by setting the client object request ID field. This permits the system to proceed without waiting for the actual picture retrieval. If user input instance [2] occurs and moves the request off-screen, the map of the multimedia document will so indicate. The media server module checks its list of client events by picture ID, to try to locate the client object associated with the client instance that moved off-screen. With the picture ID and with the request ID, the module knows how to cancel the outstanding picture request. The request ID provides a binding up of the multimedia instance [1] and its processing. The present invention further provides a flag that queries "is request complete"? If the request is complete (state E) , then the request ID is no longer meaningful, there is no need to concern the system with cancellation, and the only required action is to release resources (state F) . As noted, at a minimum between steps (4) and continuing through step (6) , a lock mechanism must be enabled to ensure consistent, atomic state changes. However, having created the client object at step (4) , the lock is now disabled to permit updating client module tables. From the client perspective, this completes input media instance [1], and everything that occurs hereafter occurs in another function stream, namely the media server process thread. (7) For each media incident [1], set the "Is Request Complete" flag to "no" Because the present invention is used in a true multi- tasking environment, the lock/unlock semaphore mechanism provides inter-module mutual exclusion to prevent module file states from being changed inconsistently. Essentially the state changes are bundled between periods of mutual exclusion. The client-server model is highly encapsulated such that while various services are provided, the media server module has no knowledge of why the services are provided. It will be appreciated that the requirement for an inter-module mutual exclusion mechanism arises because of the versatility inherent in the present invention. By contrast, a single thread application would by definition involve but a single process running in a single module, and would not require any external locking mechanism. Further, a single thread system would by synchronous and could simply call into a picture transfer mechanism that would instantly act upon the call. (8) For each media incident [1] the client module is called back Steps (8) to (11) describe what happens to the client module server thread when transition to state E occurs. Within the multi-tasking environment, the process thread that does the reading is the server task. According to the present invention, the server task advises the client module via a callback routine (a function call) that the requested picture is now complete, whereupon the client module knows what action to take. Thus, the conclusion of event [1] in the normal case will be that the user module is called back with the callback routine. Step (8) is asynchronous in that, having made the request, the client thread simply continues execution. At some point after the original client request, the request is actually processed in the server thread, which callbacks into the client module to advise completion of the request. Because step (8) is asynchronous, the callback function is required to advises the client module that the picture is done. Further, because multiple requests can be outstanding, the client callback must be passed either the request ID or the client state object to advise the client module which picture request has been completed. This server thread is setup to accommodate resource and time expensive actions, thereby freeing the client thread from such activities. This permits the client thread to handle items that are less expensive from a user-resource perspective, such as modifying user states based upon user inputs. Again, it undesired that the client module be required to handle details associated with the drawing of a requested picture. The client module is reentered in the server thread and notified via the callback routine that the request is complete. When the copying from server memory to client memory has occurred and state C to state D transition is complete, the VFILE media server has a pointer to the callback function (essentially a function call) . The pointer which returns to the client module, while retaining the current thread. It will be appreciated that simultaneously the client thread could be requesting another picture, and be running through the module while the server task is completing the picture request. It is this aspect of the present invention that requires a mutual exclusion mechanism that is provided by lock/unlock semaphores in the preferred embodiment. (9) Look for Completed Picture in Client Object List By way of assertion checking, upon completion of the picture a confirmation is made that the completed picture is in fact on the user's list of client objects. If not, the callback routine fails because an error has occurred, which error is logged whereupon the server proceeds as if no error had occurred. Such error could result from careless software making a false assumption that cancellation had occurred, and clearing resources. The error could also result from a second user attempting to cancel the request without first checking a return value to see whether the cancellation is allowed. The second user might then simply assume cancellation had occurred and clear resources, while a different user is requesting the same object. Such error could also occur if upon completion of a first user's request, a second user has already deleted the same picture from the client object list. Generally in the case of such errors, the video output memory will probably be invalid, whereupon state C will be trashed, or the video output memory will have been orphaned (e.g., the only reference to the memory was in an object that has been freed) . It will therefore be appreciated that such assertion checks are necessary to provide robust software in the present invention. Thus, before examining the client object list, it is necessary to enable the lock to prevent another user from altering the client object list while it is being examined. (10) Check for "Must This Request Be Killed" Flag The second flag, "Must This Request Be Killed" is necessary to account for the critical edge case referred to earlier. If the cancel request cannot be immediately completed, the cancel request "fails", and the second flag is marked "Yes", indicating that at some appropriate time that will not crash or otherwise endanger the system, the request must be killed. If the object must in fact be killed, resources may be released and the object deleted from the object list, after which the lock may be disabled. (11) Map Client object to idget After confirming that the client object is on the client list, the client object is mapped to a widget. The VWin service tells the system where the object is to be drawn, and VWin preferably is always made drawable, for example permitting XWidget to be drawn upon request, in black on a white background. As noted, in the preferred embodiment, the graphics board permits drawing a layer of color over the video image, which does not appear on the display because of the color. (12) Request Draw "Clear" into Widget As noted, drawing requests have been made asynchronously. In the absence of a cancellation, a request is now made to draw "clear" into widget, which means filling an array in graphics memory (but not on-screen) with a particular number. In the graphics table associated with video board 26 (see Figure 2) , the number 0=clear, l=white, 2=black, 3=blue, 4=red, and so on. Thus by filling the graphics memory array with O's, the opaque colored overlay atop the previously drawn video is removed. (13) Mark Client Object as "Done", Mark "Is Request Complete" Flag "Yes" At this point, the request has completed, the client object is marked as done, and the "Is Request Complete" flag is set to "yes". The previously drawn image now appears on display 22 (see Figure 2) . At this point, the server resources used to complete the now displayed request are unlocked and released. What will now be described is the same instance [1], but as viewed from the server module. To avoid ambiguity in cross-referencing the descriptions, the steps will continue to be numbered serially. However it must be understood that while, for example, step (14) in the following description is a higher number than step (13) in the above description, it in fact occurs earlier in time. Instance m as viewed from the server module: (14) Client Thread Calls into Picture Transfer Request Service A call is made by the client thread into the picture transfer request service. (15) Create New Request Item and Request ID After first enabling the lock mechanism, a new request item and request ID are created and video memory is allocated. At this point, the client thread is still at state A. The requested queued object has several attributes, some of which have been described. There is an associated picture ID, a callback routine used for every queued object, an associated client state, a function state that can be "inactive", "in process" or "locked", a "must request be killed" attribute and preferably a priority attribute. A client state is needed to support the callback routine, which in turn is needed because of the asynchronous request mechanism provided by the present invention. The "inactive" state denotes that the request is still sitting in the request queue with no action having been taken yet. "In process" denotes that some action is being taken with respect to the request, and the nature of the action will allow interruption or cancellation. The "locked" state denotes that the request is at a process stage that precludes interruption or cancellation at this time. Because the present invention permits cancelling a queued request that has not completed, requests may be prioritized and even re-prioritized at any time they in the queue. This aspect of the present invention enhances overall system performance. Priority can be assigned according to expense of system performance (e.g., relative amount of video memory required for the image, relative time to complete the image) , and/or according to user benefits (e.g., how important is it to now view a fully rendered image as opposed to a wireframe image, or a black and white image as compared to a full color image) . High priority requests (those requiring less media resources/time) can be queued before lower priority requests. As an alternative to assigning a priority number per se, the picture ID enables the server module to guess whether the picture should be rendered fast or slow, by querying the segment library as to the file size. Requests for both high and low priority images can be made, by the user directly or by software less directly. The high priority image might be a request for a wireframe depiction of a building, whereas the lower priority request may be for the fully rendered building image. Both requests are queued and the wireframe request will complete processing first. If the user "runs" rapidly past the wireframe-rendered building image, there is no need to complete the now obsolete request for the fully rendered image of the already passed building. The present invention cancels that request and goes on to the next request. Intelligent software permits the present invention to pre-queue scenes likely to be visited or requested by the user, which requests if not needed may be cancelled before they are complete. In this example, the present invention can automatically adapt the quality of the displayed images to the navigation speed of the user, showing wireframe images during rapid navigation, showing better rendered images when slowing or halted. In this fashion, the present invention is adaptively controlled to keep up with the user navigation as best it can. System throughput is enhanced because cancelled requests make system resources available to other requests. This in turn permits the user to rapidly and freely navigate the multimedia space, essentially with minimum real-world constraints. (16) Save Request ID on Stack (or Set Return Value) The just created request ID is entered into a memory stack as it is desired to pass the request ID out of module state space, which space can now be unlocked. It will be appreciated that once unlocked, nothing should be referenced within the module. At a later time, the request ID will be referenced (and thus required) , namely when it is returned (see step (6) , above) . At this point, a return value is set. Alternatively, a pointer could be passed to a space that held the request ID. (17) Wakeup Server Thread The presently executing thread is the client thread, and it is necessary to wakeup the server thread. In the preferred embodiment, the server thread normally runs but is blocked from continuing to run by a semaphore. The wakeup advises the server thread that a new item has entered the request queue. This semaphore blocking/unblocking is preferable to an implementation wherein the non-desired thread is kept in a "busy waiting" state, wherein it constantly had to use computer 10 CPU resources. In the present invention, the semaphore is a lock-type mechanism provided within any kernel that provides multi¬ tasking as a procedural call, typically denoted SemTake, SemLock, etc.. Essentially the semaphore knows only "yes", "no", and who owns it. Semaphores in the present invention allow one process to execute while another process is in a wait state. Eventually the client will add an item to the request queue, at which point the client raises a semaphore that wakes-up the server thread, alerting it to the existence of at least one newly queued request. In the preferred embodiment, the server is defined as having a lower priority than the user, so as not to unduly stress the system when a picture is to be drawn. When the client wishes to raise the semaphore to the server thread, this makes the server runable. When the client process next pauses, for example to wait for more user input, the server can begin processing requests. This represents a scheduling aspect of multi-tasking in the present invention, in that the next highest priority runable task is permitted to run. (18) Return At this point, the client thread returns. (19) Server Thread Waits for Request Having been awakened at step (17) , within the server module the server thread now runs, and checks the request queue. (20) WakeUp Arrives Sometime after the wakeup arrives, the next request will be fetched from the queue by the server thread, although not necessarily immediately, depending upon other processes in the system. At step (20) , the module is locked. (21) Get Next Request From Queue After the user event [1] created the request at step (1), the request was added to a list of requests at step (5) . To ensure consistent state changes, the lock is enabled before step (21) . According to the present invention, the requests to be queued may be prioritized, and queued according to priority such that faster executing requests are ordered before slower executing requests (although other ordering or priority considerations may govern) . Sometime after the walk-up arrives, the server thread will be able to proceed but in the interim remains in a pending state without having to loop or engage in other activity that would require extensive use of the computer 10 CPU. (22) Mark Request as in Stage 1 Processing As indicated by Figures 8A, stage 1 processing implies the request will be using server resources (but not client resources) . The "in process" mark implies that the picture request is being processed. The server thread proceeds to obtain the requested picture from the media server, without locking module resources at this point. It is understood that the lock is enable when marking the requested object, but is disabled before the relatively expensive media read proceeds. (23) Read From Disk or Other First Stage Media At this point, the server thread reads from the first stage media server resource (e.g., hard disk 18 in Figure 2) , and brings the retrieved information into server module resources to make the information available. During the relatively slow file input/output process, the lock is disabled. When the file input/output process is complete, the data is within server module memory. As we move through the server thread handling the requests, step (23) is a point whereat the requested object may actually be examined. The lock is enabled and, as will be described, at step (24) the process determines whether the request is still active, and further checks to see if the request has been killed. It should be noted that the low level facilities of the operating system are provided by commercially available software. For example, VXWorks provides some file input/output facilities, and scheduling and semaphores facilities. As noted, XWindows is similarly a commercially available product, known to those skilled in the relevant art. (24) Check to See Whether Request Is Still Active Step (24) describes how the server module, within the server thread, actually proceeds through the process of handling a request. Because determining whether the request is still active involves a branch operation, the module is locked for the duration of this step. If the request is still active, we proceed to step (25) . If, however, the request is no longer active, then the server module considers itself done with the request, and server resources previously allocated for the request are released. The lock is disabled, and the process returns to step (21) to see whether there are additional outstanding requests in the queue. It is not required that step (19) be re-traversed before reentering step (21). When a request is cancelled during actual processing of the request, the request on the server side will be marked as deleted but will remain in the queue. To avoid inconsistent module state, server side resources are not freed at this point. Understandably a race condition could result if while a request file was in process, the server memory into which the file is to be copied is itself freed. Further, it must be recognized that the calling thread wishing to cancel is not the server thread but the client thread. (25) Mark Requested Object as Uninterruptible The object whose request is still active is now marked as "uninterruptible" and the lock is disabled, which permits read access to the client object list. However because the request is marked "uninterruptible", no action can occur in response to anything on the list. Note that this action is solely within the server module. The only client access into the module is via well defined procedural calls, and there need be no reliance upon the client module not altering server module states. (26) Copy Requested Object into Client Resources At this point, the requested object is copied into the video output memory, for example into memory 26 in Figure 2. Upon completion of the copying, the lock is enabled. (27) Get Callback Pointer Associated with the queued object whose request is still active is a callback pointer that is set as a field within the picture request. (28) Callback Is Called At this juncture, the lock is disabled and the callback function is called, with the client state. Again, it is the callback function that facilitates the communication, which is inter-module rather than inter-process. What will now be described is user instance [2], as viewed from the client module. As before, for the sake of avoiding ambiguities, the steps will continue to be numbered serially. It should be noted that event [2] occurs entirely within client thread(s) and never within any server thread. Event [2] is simply another event along the execution stream, namely a branching from the end of the thread in event [1]. Instance 2 ~ as viewed from the client module: (29) User Input Causes Action to Move User Context/Reference Point within the Multimedia Space As has been noted, user input might be a client thread representing mouse or trackball movement, or it might be a different client thread representing a software recognition that a velocity or acceleration threshold has been exceeded. In any case, such event [1] results in an action moving the user context or reference point within the multimedia space being navigated. (30) Client Software Detection that certain Active Events Have Moved Out of the Active Region Based upon the used instance or event [1], client software within the calling client thread detects that certain active events have moved outside the active region on display 22. The active region is client defined, and may in fact be arbitrarily defined with several levels. The active region could be what is displayed, or perhaps what is displayed plus what the system wishes to retain for performance reasons. A system providing heuristic software function might pre- request pictures representing scenes adjacent to what is currently displayed. (31) For Each Such Event, Search Picture Object List For Event The lock is enabled, and the list of picture objects is searched for the event being requested. If not found, the lock is disabled, an appropriate error message is generated, and the process loops back to step (31) , where a similar assertion check is made for the next such event. (32) For Each Such Event Found on Picture Object List, Check Picture State If, however, the picture object list contains the event (as it normally would) , the state of the picture is next examined. If the active region has already been drawn (e.g., state E) , then by definition the request is complete, resources may be released, the object request is deleted from the queue, the lock is disabled, and the procedure loops back to step (31) to search the object list for the next such event. However If not yet drawn (e.g., the request is somewhere within the server queue, between states A and D) , the cancel request routine in the server module is called. The call is made with the request ID that was saved with the description of the client object corresponding to the multimedia event. (The client object also contained the picture ID that was passed to the picture server.) A cancel request success flag is returned that is either in a "yes" state or a "no" state. A "yes" means that the request was either successfully cancelled, or that the cancel request was noted. A "no" means the request cannot be cancelled, and the process continues to step (33) . Because of the encapsulation provided by the present invention, the client module has no concern with why the request was or was not cancelled. If the cancellation succeeded, the process returns to step (32) for the already drawn state, whereupon client resources are released, the object request deleted from the queue, etc. As far as the client module is concerned, the request is now cancelled. It is noted that while the client module is free to immediately release its resources, the server module may defer cancellation (and resource release) until a suitably non- critical time. The present situation is described by Figure 9, state A wherein cancellation succeeds because the request was still inactive, or by Figure 10, state B, wherein cancellation succeeded because client resources had not yet been involved. from perspective of client, safe to cancel. Thus, as shown in Figure 9 and Figure 10, from the client perspective, it is safe to now release all system resources. (33) For Each Such Event Found on Picture Object List For Which Cancellation Is Denied, Mark Object "Must Be Killed" If the success flag call returns a "no", then the request object sought to be cancelled cannot at this instant be cancelled. It is, however, marked or flagged as "Must Be Killed", which flag enables the request to be killed within the server module at an early non-critical time, e.g., a time whereat the system will not be crashed. Beyond this, the client thread can do no more. With reference to Figure 11, such a "no" return will result after state C. However, by the time the process reaches state E, the pending picture request will be killed. The kill time will be when the server thread calls the callback, at which time rather than draw the object, it is killed, resources deleted, the object deleted from the client object list. (34) For Each Such Event, Unlock and Loop Back to (31) At this point, the process returns to step (31) , and searches the list of picture objects for the next detected event [2]. As will be seen from the following description, the marked and pending kill request will be accounted for the next time the callback function issues out of the server thread. In essence, because of the asynchronous nature of the present invention, the system can rely that at some later time, another thread will enter the client module and complete the pending cancellation. What will now be described is user instance [2], as viewed from the server module, the process steps again being numbered serially. The following actions occur within several threads, but on an intra-module basis. Instance f21 as viewed from the server module: (35) Request to Cancel Request by Request ID Returned from Request to Transfer Service In the line of execution, there is a call within the client thread whereby the server module provides a cancel request to the user interface, which request is cancelled by the client thread. See step (32) , undrawn case. In making the cancel request, the cancel request routine is passed a request ID, which ID was returned when the client originally requested the transfer. (36) Locate Request in Queue On the server side, the lock is enabled to preserve the server module states, and the outstanding request is located in the queue by its ID. (37) If Request Not Found, Error Case The request should of course be in the queue. However if this assertion check indicates otherwise, the lock is disabled and an error message generated to the log. The error could have resulted from a callback that was advised that the request had succeeded, but the state was not marked. At a later time the caller might try to delete the request, which request may have been long since deleted by the system. (38) If Request Found, Check State of Request In the normal course of events, the request should be found in the queue, whereupon the request state is checked with the lock still enabled. If we are at inactive state A, e.g., in the queue but inactive, the request is deleted from the queue and server resources are released. The lock is disabled, and a cancellation "success" is returned. This is the case depicted in Figure 8A, where the request never got around to start of processing. If we are at active state B, then the server object on the server side is marked as "must be killed", the lock is disabled, and a cancellation "success" is returned. It should be noted that the server had not yet deleted the object, but when the server transitions from state B to state C, a check is made to see whether the request was still active. If the request must be killed, server resources may be safely released at this point and the system proceeds to process more requests. If the request is uninterruptible, the lock is disabled, and a "fail" is returned. An uninterruptible server request represents the case where the server module cannot back-out of the process. An uninterruptible request may represent state C, or if the server process has commenced, the more critical state D. To prevent deadlock, the server module unlocks its server module lock before calling the callback, because otherwise the client thread could enter and lock the server module. Thus in state D, the server must relinquish its lock when calling the client callback, and in fact it is this action of calling the client callback that transitions from state D to state E. However before unlocking the server module and calling back, the server thread marks the object called for as "uninterruptible". Thus, even if the server thread blocks on the client lock in the callback, the client thread will not alter the current request within the server module. When the server thread eventually proceeds with the callback, (after the client thread releases the client lock) , the picture request completes, and the server moves on to any other pending requests. Modifications and variations may be made to the disclosed embodiments without departing from the subject and spirit of the invention as defined by the following claims. For example, while the present invention has been described with reference to application with a video editor system, it will be appreciated that any application requiring retrieval and presentation of media will benefit from the present invention.

Claims: WHAT IS CLAIMED IS: 1. In a computer system including a main computer having a memory, at least one media server coupled to the computer and having a memory for storing information, generating means coupled to the computer for generating media access requests, and an output device coupled to the computer, a method for asynchronously executing media access requests to the media server for outputting said information over said output device, including the steps of: generating, in said generating means, an access request to said media server; placing said generated access request in a queue of media access requests; processing said access request when it reaches a top of the queue, including retrieving a portion of said information from said media server and providing said retrieved information to said output device; if a cancellation request is received during the processing step, canceling the access request by removing the request from said queue and releasing resources which are accessed during said processing, including said media server, if said resources are currently being accessed by said access request, the canceling step including implementing a cancellation delay procedure if the cancellation request is received during a predefined procedure in the execution of the media access request. 2. The method of claim 1, further including the step of assigning a priority to each said media access request; wherein said placing step includes placing said media access request in said queue at a position corresponding to said assigned priority. 3. The method of claim 2, wherein said assigning step is carried out based upon predetermined system resource allocation criteria. 4. The method of claim 2, wherein said assigning step is carried out based upon previously generated user input events. 5. The method of claim 2, wherein said assigning step includes the step of comparing each said priority to previously generated priorities assigned to previously queued media access requests. 6. The method of claim 1, wherein said generating means comprises a user input device coupled to the computer. 7. The method of claim 1, wherein said generating means comprises program logic stored in said computer memory. 8. The method of claim 1, wherein said predefined procedure is governed by a server program module of said media server and controls a resource managed by a client program module stored in said main computer memory. 9. The method of claim 8, wherein said predefined procedure comprises the loading of said portion of said information from said media server memory into said computer memory. 10. In a computer system including a main computer having a memory, at least one media server coupled to the computer and having a memory for storing information, generating means coupled to the computer for generating media access requests, and an output device coupled to the computer, a method for asynchronously executing media access requests to the media server for outputting said information over said output device, including the steps of: (1) generating, in said generating means, an access request to said media server; (2) assigning to said access request a priority; (3) positioning said generated access request at a first position in a queue of media access requests, said first position being dependent upon said assigned priority; (4) processing said access request when it reaches a top of the queue, including retrieving a portion of said information from said media server and providing said retrieved information to said output device; and (5) if a reprioritizing request is received during the processing step, assigning a new priority to said access request based upon said reprioritizing request; wherein step (5) includes implementing a reprioritizing delay procedure if the reprioritizing request is received during a predefined procedure in the execution of the media access request. 11. The method of claim 10, wherein: said reprioritizing request comprises a request for changing the priority of execution of said access request relative to other access requests; if said access request is not currently being executed, step 5 includes the steps of removing said access request from said first position in said queue, and repositioning the access request to a second position in said queue corresponding to said new priority; and if said access request is currently being executed, said reprioritizing request is not carried out. 12. The method of claim 10, wherein: said reprioritizing request comprises a request for cancellation of said access request; and step 5 includes the steps of: removing said access request from said first position in said queue; releasing resources which are accessed during step 4, including said media server, if said resources are currently being accessed by said access request; and discarding the access request after said removal from said queue. 13. An asynchronous media server request processing system for presenting information from media servers in response to media access requests, including: a main computer having a main memory; at least one resource accessed in response to said access requests, including at least one said media server coupled to said main computer and having storage means for storing information; an input device coupled to said computer for user input of said media access requests; first program means stored in said main memory for assigning a first priority to a first said media access request; means for generating a reprioritizing request for assigning a second priority to said first media access request; an output device coupled to said computer for outputting said information; and second program means, stored in at least one of said main memory and a memory of one said media server, for executing said media access requests, including: a client module for generating said first media access request and defining a client object for each said access request; a media server module for: generating a queue including a plurality of additional said access requests; placing said first media access request at a first position in said queue, said position depending upon said first priority; removing from said queue access requests which have been executed; controlling said at least one resource during the execution of said first media access request; and outputting said information to said output device upon said execution; means for implementing said reprioritizing request by removing said first media access request from said first position in said queue and releasing any said resource which is currently controlled by said server module; and means for delaying said prioritizing request implementation if said prioritizing request is generated during a predefined portion of the execution of said first media access request. 14. The system of claim 13, wherein: said reprioritizing request comprises a request for assigning said second priority to said first media access request, for changing the priority of execution of said first media access request relative to other said media access requests; and said implementing means includes means for repositioning said first media access request to a second position in said queue corresponding to said second priority. 15. The system of claim 14, including means for preventing said implementing means from carrying out said reprioritization request during execution of said access request. 16. The method of claim 10, wherein: said reprioritizing request comprises a request for assigning said second priority to said first media access request, said second priority corresponding to a cancellation of said first media access request; and step 5 includes the step of discarding said first media access request after said removal from first position in said queue. 17. The method of claim 13, wherein said reprioritizing request generating means comprises a user input device coupled to the computer. 18. The method of claim 13, wherein said reprioritizing request generating means comprises a portion of said second program means.

Inventors: JAUHAL SHERN, SCHMITZ PATRICK L, JAUHAL, SHERN, SCHMITZ, PATRICK, L.
Applicants: ACCOM INC, ACCOM, INC.
Application year: 1994
Main classification: G06F12
IPC codes: G06F13